This invention relates to gas discharge lamps and systems.
The invention is more particularly concerned with gas discharge lamps and systems including control circuits for such lamps that enable them to be dimmed.
Gas discharge lamps, such as fluorescent lamps, have advantages of being able to produce high intensity light with a low power dissipation, they also have a relatively long life and are relatively robust. One disadvantage, however, with these lamps is that their brightness cannot be well controlled over a wide range. Most commercially-available fluorescent lamp dimmers operate by varying the mark-to-space ratio of the drive signal applied to the lamp and can only control brightness over a range of about 150:1, although some dimmers for aerospace applications are capable of controlling light output in the range 2000:1. In U.S. Pat. No. 5420481 there is described a fluorescent lamp system having glow mode electrodes in addition to the two normal electrodes at opposite ends of the lamp. These electrodes extend externally along the length of the lamp and are used to produce low level light output, which enables the output range to be considerably increased. One problem, however, with these glow mode electrodes is that any irregularity or misalignment in the walls of the lamp can lead to a non-uniform field from the glow electrodes and, hence, to a non-uniform light output. Some fluorescent lamps used to backlight displays are bent into a serpentine shape, so as to give a more even illumination over the surface of the display. This form of lamp presents substantial problems in achieving an even illumination using the previous form of glow mode electrodes.
EP-A-653903 describes a fluorescent tube in which one of the discharge electrodes is connected to the end of a single helical wire extending about the tube. As voltage is increased, the discharge moves gradually along the length of the tube. GB 2305540 describes a system for dimming fluorescent tubes by applying a voltage between two parallel helical wires, so that the voltage is applied across the diameter of the tube. This latter arrangement can enable a very good control of dimming at low intensities.
Where displays or the like are backlit, it is necessary to produce even illumination across the display. To produce even illumination efficiently using tubular lamps requires complex and expensive diffuser optics. It is also expensive to bend the tubular lamps by hand into a serpentine shape. Because of this, there has been a move towards using flat panel, planar, fluorescent lamps. Such lamps comprise a box-like structure with parallel internal walls defining a serpentine, or similar path between the two cathodes. These planar lamps can produce an even illumination across their surface at relatively high levels of illumination but the illumination becomes more uneven at low levels. It has been found that, at low levels, there is a tendency for the discharge plasma to adhere to the dividing walls within the lamp. In aircraft applications, it is particularly important, for night-time viewing that there is an even illumination across the display at low brightness levels.
It is an object of the present invention to provide an improved planar gas discharge lamp and system.
According to one aspect of the present invention there is provided a gas discharge lamp including a planar envelope filled with a discharge gas, means defining an elongate discharge path within the envelope, and a first pair of electrodes located at opposite ends of the discharge path for causing discharge within the envelope, the lamp including two glow mode electrodes spaced from one another across the discharge path so that a voltage can be applied between the glow mode electrodes across regions of the discharge path.
The discharge path is preferably a serpentine path defined by a plurality of parallel walls extending part way across the width of the envelope. The glow mode electrodes preferably extend in alignment with the walls and along the same surface of the envelope. The glow mode electrodes are preferably on a front side of the envelope through which light is transmitted. The glow mode electrodes may take the form of two interdigitated combs. Alternatively, the glow mode electrodes may extend along opposite surfaces of the envelope. The glow mode electrodes preferably extend on an external surface of the envelope and may be of a transparent material.
According to another aspect of the present invention there is provided a gas discharge lamp system including a lamp according to the above one aspect of the invention, a first circuit for driving the first pair of electrodes and a second circuit for driving the glow mode electrodes.
The system is preferably arranged such that at high brightness only the first circuit drives the lamp, at low brightness only the second circuit drives the lamp, and at intermediate brightness the first circuit drives the first pair of electrodes for successive periods separated by spaces and the second circuit drives the glow mode electrodes only during the spaces between the periods when the first pair of electrodes are being driven. The first circuit is preferably arranged to reduce the space between the periods of its output, the second circuit being arranged to increase the space between the periods of its output when an intermediate brightness of the lamp is to be increased. The second circuit is preferably arranged to drive the glow mode electrodes at a frequency of the order of at least 5 MHz, which may be of the order of 10 MHz.
According to a further aspect of the present invention there is provided a method of driving a gas discharge lamp of the kind having a planar envelope filled with a discharge gas, means defining an elongate discharge path within the envelope, a first pair of electrodes located at opposite ends of the discharge path for causing discharge within the envelope and two glow mode electrodes spaced from one another across the discharge path so that a voltage can be applied between the glow mode electrodes across regions of the discharge path, the lamp being driven by energizing only the first pair of electrodes at high brightness levels, energizing only the glow mode electrodes at low brightness levels, and at intermediate brightness levels energizing the first pair of electrodes for successive periods separated by spaces, and energizing the glow mode electrodes only during the spaces between the periods when the first pair of electrodes are driven.
The method may include driving the glow mode electrodes at a frequency of the order of at least 5 MHz. The method may include the steps of reducing the space between periods of signals supplied to the first pair of electrodes and increasing the space between periods of signals supplied to the glow mode electrodes when an intermediate brightness of the lamp is to be increased.
A fluorescent lamp system for backlighting a display, in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings.